Separation devices such as casings, covers, doors, flaps or the like are frequently provided in dangerous machines to prevent persons from accessing danger zones of the machine during operation. It can furthermore be necessary for the operation of the machine that specific safety-relevant machine parts are in a predefined safe state. To ensure that the separation devices are actually closed or that the safety-relevant machine parts are in their safe state during the operation of the machine, safety devices are provided that only permit a putting into operation of the machine when the separation apparatus are closed or are in a safe position or when the safety-relevant machine parts are in their safe state. An opening or removal of the separation apparatus during the ongoing operation of the machine as a rule results in an immediate switching off of the machine.
For example, an actuator can be arranged as the object to be detected at the separation device that is only located in the monitored zone of the sensor and is only detected by the sensor when the associated separation apparatus is in its safe position in accordance with its intended purpose. An opening or removal of the separation apparatus has the result that the actuator is removed from the monitored zone and the sensor thus no longer outputs any detection signal which in turn has the result that the evaluation unit outputs a corresponding output signal. This output signal can, for example, be an opening of a safety switching contact. This safety switching contact can be monitored by the machine, with an operation of the machine only being permitted with a closed contact.
With such a safety device, a so-called proximity switch, in particular an inductive proximity switch, can, for example, be used as a sensor. A sensor configured as an inductive proximity switch generates a magnetic field in the monitored zone. If a metallic object, in particular a ferromagnetic object, is located as an actuator in this magnetic field, it effects a change of the magnetic field strength that is detected by the sensor and that initiates the output of the detection signal.
Conventional safety devices are, however, frequently manipulated. In order, for instance, to enable an operation of the machine with a removed separation device, a non-system actuator or simply another metallic object is introduced into the monitored zone of the sensor instead of the actuator arranged at the separation device and a safe state of the machine is thus incorrectly simulated. The danger of such manipulations is also present in a corresponding manner with respect to the safe state of safety-relevant machine parts.
Coded actuators can be provided to prevent such manipulations. An example for such systems are so-called RFID (radio-frequency identification) systems in which an RFID transponder is used as an actuator and an RFID reader is used as a sensor. The RFID transponder transmits a characterizing code that enables an identification of the detected object or actuator and thereby makes manipulations more difficult. However, non-coded safety systems (such as inductive sensors) and also low-level coded safety systems (such as magnetic sensors and RFID systems in a universally coded operating mode) can be manipulated since the number of available code variations is frequently limited and thus objects having the same code (e.g. with the same pattern or the same range of RFID numbers) are used for manipulation. Systems having a higher code level are frequently substantially more expensive.
In addition, with all coded safety devices, an increased effort is required on a new installation or on a replacement of sensors and/or actuators since the codes of the sensor and of the actuator have to be matched to one another and a multiple use of the same code in a respective machine should be avoided.
DE 203 06 708 U1 relates to an access protection device with whose aid an access opening of a machine can be monitored that is closable by a door. The door can be latched by means of a bolt that comprises a magnetizable yoke or a magnet closable by a yoke. The closed state of the door is monitored with the aid of a magnetic field sensor, with a sensor element additionally being provided that can receive signals from a transponder attached to the door.
A similar apparatus is described in DE 102 52 025 A1. The door is secured with the aid of a lock that has a latch and a bolt for holding the latch in its closed position. Two transponders are fastened to the latch or to the bolt. A sensing head can then only receive identification signals from the two transponders as a detection signal or as an additional criterion when the latch is in the closed position and is held by the bolt.
A further apparatus of a similar category is disclosed in WO 03/012998 A1. Here, the closed state of a door is monitored with the aid of a Hall element, with a first output signal being generated in dependence on the presence of a magnetic element. In addition, a window comparator is provided that monitors the output signal of the Hall element as to whether it is located within a predefined signal window, i.e. whether a second higher signal threshold is not exceeded. A manipulation with any desired magnets should be precluded by this additional criterion.